Gas turbine engines operate over a broad range of speeds and thrusts, and as a result generate a broad range of noise frequencies. Acoustic treatments in the form of acoustic liners that line the fan and exhaust ducts of gas turbine engines are widely used to suppress aircraft engine noise beyond those levels that can be achieved by the particular design of the turbo machinery. In view of stringent noise abatement requirements around the world, considerable effort has been directed to designing acoustic liners that are capable of absorbing noise over a broad range of frequencies, while also being durable, relatively low-weight, readily fabricated, and having minimal impact on engine performance.
There are two primary sources of aircraft-generated noise. One source is the viscous shearing that takes place between the rapid exhaust gases and the relatively quiescent surrounding air, while the second source is the rotating blades of the fan, compressor and turbines, and the resulting air flow past the vanes and other stationary objects within the engine air flow path. Acoustic treatments for suppressing noise produced by the latter source can generally be categorized as bulk absorbers or resonator-type absorbers.
A bulk absorber 10 is represented in FIG. 1. With this type of treatment, a porous material 12, such as a fibrous or rigid foam material, fills a cavity between two sheets 14 and 16. The sheet 14 is formed of an air-permeable material that forms the walls of a nacelle flow duct of a gas turbine engine, e.g., the fan inlet and fan exhaust ducts and the turbine exhaust duct. The sheet 14 and the bulk absorber 10 absorb sound waves that impact these walls as the waves propagate through the duct. Examples of suitable materials for the sheet 14 include sheet fabricated from sintered or felted metal, or other porous materials having suitable flow resistances. The back sheet 16 is typically rigid and air-impermeable.
Acoustic treatments referred to as resonator-type absorbers include Helmholtz resonator chambers or compartments. A double-layer resonator absorber 20 of this type is represented in FIG. 2 as having a compartmented airspace core with an air-permeable facesheet 22 and an air-impermeable back sheet 24, between which there are a number of compartments or cells 26. The facesheet 22 typically has perforations 30 within which sound absorption occurs. In the double layer resonator 20 shown in FIG. 2, a porous septum 32 is present between and parallel to the facesheet 22 and back sheet 24. Conventional methods by which the resonator 20 is manufactured typically entail individually forming the resonator layers separated by the septum 32, and then bonding the layers and the septum 32 together. As a result, misalignment often occurs between the cells 28 of these layers. In a single-layer resonator (not shown), the porous septum 32 is omitted.
As a rule, the cells 26 of resonator-type absorbers have been defined by hard, air-impermeable walls 28, which are often configured so that the cells 26 have a hexagonal-shaped cross-section that yields a honeycomb cell pattern. Passages between resonator cells 26 have been proposed, as shown in U.S. Pat. Nos. 3,972,383 and 4,189,027. However, the former resonator absorber relies on air being forced through the cells 26 from an exterior source in order to tune the facesheet 22, while the latter absorber requires adjacent cells 26 to be asymmetric, which causes air pumping between cells 26 when air flows over the perforations 30 in the facesheet 22.
There are known advantages and shortcomings with each of the acoustic treatments described above. The double-layer resonator-type absorber 20 represented in FIG. 2 provides good noise attenuation over a relatively wide band of frequencies centered about a particular frequency to which the cells 26 are tuned, based in part on their depth. To achieve a broadband capability, a resonator-type absorber must have a variety of cavity sizes to cover the frequency range of concern, or must be capable of mechanically changing the sizes of the cells. Both of the approaches are mechanically complex and contribute undesirable weight to the engine.
In contrast, bulk absorbers of the type shown in FIG. 1 offer higher suppression performance than either single-layer or double-layer resonator-type treatments by their ability to absorb noise over a wider frequency range. In spite of this performance advantage, bulk absorbers are not widely used in aircraft engines due to disadvantages inherent in she material properties. Specifically, the conventional concern is that fibrous materials will disintegrate with aging and the high dynamic vibration levels within gas turbine engines, and may wick liquids that could create a fire hazard. Another drawback of bulk absorbers is their poor serviceability.
In view of the above, it can be seen that it would be desirable if an acoustic treatment were available for gas turbine engines, by which a broad band of noise suppression was possible along with structural integrity compatible with air flow conditions of the gas turbine engine environment.